The present invention relates to a telescopic part, more particularly, for the jib of a crane or mobile crane, having a closed cross-section of composite materials. More specifically, the present invention relates to a telescopic jib for a crane or a mobile crane, including an articulately jointed base section and at least one telescopic section formed from the composite material.
Telescopic jibs, as employed for instance on stationary or mobile cranes, are configured of several nesting telescopic sections which can be extended to elongate the jib. Each telescopic section is mounted to slide on the other. One factor salient to the loading capacity of the individual sections is the consistently straight cross-section of the telescopic parts.
This dimensional fidelity is ensured by the material properties of the telescopic parts and, on the other hand, by end frames which are required to exhibit a corresponding stiffness, and to serve to introduce the forces into the individual telescopic sections. These end frames are generally termed collars.
Conventional optimized jib cross-sections are fabricated usually of high-strength, weldable, fine-grain steels. The dead weight of the jib, which is relatively high in the case of steel designs, plays a significant role since, on a long reach, most of the loading capacity of the cross-section has already been used up in carrying the dead weight. This is why steel telescopic parts are basically too heavy, but are used typically in prior art due to the high strength of steel.
Known from EP 0 117 774A1 is a telescopic jib comprising telescopic parts featuring a core of expanded polyurethane covered by a skin of a composite material or of aluminum. However, despite its stability being relatively good due to the structure involved, such a sandwich design has inadequate strength for long telescopic jibs in heavy loading situations.
The object of the present invention is to provide telescopic parts/jibs optimized in weight and strength.
This object is achieved in accordance with the invention by the telescopic part comprising a composite cross-section of a layer of steel and at least one layer of a fiber composite.
In accordance with the invention part of the fine-grain steel cross-section conventionally employed is thus replaced by a fiber composite layer exhibiting, for the same strength and stiffness, a significantly reduced specific weight. The ratio of the loading capacity to the dead weight becomes all the more favorable, the higher the modulus of elasticity of the composite.
A further advantage afforded by the telescopic part in accordance with the invention is rooted in the fact that jib oscillations are reduced. Fine-grain steel jibs have such low natural frequencies that resonance may be prompted simply. by operation or by the wind. Due to the better damping performance of the fiber composite layer employed in accordance with the invention such resonance can be suppressed and the jib quickly comes to rest, it being not possible in general for oscillations to be generated as long as the layers are sufficiently thick.
Yet a further advantage afforded by the telescopic parts and jibs in accordance with the invention is the low deformation due to heating up when exposed on one side to sunlight, which results in undesirable high deformations in the case of steel telescopic parts which, in turn, diminishes the loading capacity.
When, in accordance with one preferred embodiment of the present invention, the steel layer forms an inner layer and the fiber composite layer forms an outer layer of the composite cross-section, the steel core of the telescopic part or jib is no longer exposed to direct sunlight, thus minimizing the differences in temperature and the resulting differences in thermal expansion in the steel. Due to the low conduction of heat and the property that plastics tend to shrink, whilst metals tend to elongate when exposed to heat, it is to be anticipated that such jibs in accordance with the invention remain substantially straighter when exposed on one side to sunlight.
Since the telescopic jib in accordance with the invention can be designed lighter for the same loading capacity, fewer counterweights are needed to compensate the moments acting in the ball bearing slewing ring of a telescopic crane.
In one preferred embodiment of the invention, the fiber composite layer comprises a first fiber composite located preferably inwardly and adjoining the steel layer, this first fiber composite featuring mainly unidirectional fibers in the longitudinal direction of the telescopic part as well as a second fiber composite located preferably outwardly and over the first layer, again featuring mainly unidirectional fibers but oriented transversely to the first layer. In this arrangement, the first and/or the second unidirectional fiber composite may be configured of unidirectional fiber mats.
In such a sandwich arrangement of the fiber composite, a mutually supported and more particularly clamping action of the first unidirectional fiber composite can be achieved by the second unidirectional fiber composite, prohibiting any pull-out of the longitudinal fibers since the transverse fibers become skew and expand, thereby, increasing the contact pressure on the first fiber composite. The longitudinal arrangement of the fibers in the first unidirectional fiber composite generates a particularly flexurally rigid structure since the fibers are expanded only in their longitudinal direction and do not need to be first pulled straight.
The first and/or second fiber composite may comprise longitudinal bundles of fibers in accordance with the invention.
Hitherto, such fiber materials optimized in weight and stability have failed to find application in engineering telescopic parts and jibs for cranes due to there being no possibility known of securing these fiber composites to the jib.
In accordance with the invention, the first fiber composite is applied and locked non-shiftingly in place to the steel layer. This can be achieved basically by one or more of the following securing options:
There is firstly the possibility of positively connecting the first fiber composite to the steel layer, i.e. preferably by extensions protruding from the steel layer engaged by the fiber composite and/or by recesses formed in the steel layer in which the fiber composite mates.
Another possibility consists of securing the first fiber composite to at least a one end of the telescopic part, more particularly to a collar, i.e. preferably by potting and/or by forming a unit securing the collar and the second fiber composite. Nested telescopic jibs have portions at the ends of the individual telescopic sections in which the flexural stresses become zero. It is in these portions in which the collars are likewise located that anchoring the fiber composite material to the steel part can be done to advantage.
There is additionally the possibility in accordance with a further securing aspect in accordance with the invention of maintaining the first fiber composite in place by the clamping action of the second fiber composite wrapping thereabove. Any pull-out of the longitudinal fibers from such xe2x80x9cwrappedxe2x80x9d fiber bundles is rendered impossible since transverse and longitudinal fibers interlock, and thus the higher the pretension in the transverse fiber and the more the pull in the longitudinal fibers, the higher is the compression. The steel part, longitudinal and transverse fibers accordingly form a positive friction connection.
In accordance with a preferred embodiment of the telescopic part in accordance with the invention, the composite cross-section and, more particularly, the first fiber composite is arranged on only part of the closed cross-section and preferably substantially in the zone of tensile loading. The tensile strength of fiber composite materials is substantially higher than their compressive strength so that it may be of advantage to arrange the first fiber composite only in the tensile loaded zone of the cross-section. The thickness of any jib shell employing a composite material is greater than that of a steel cross-section for the same weight. This results in added stability in preventing localized failures such as plate denting and shell rupture.
The second unidirectional fiber composite including fibers oriented transversely to the first composite prevents, on the one hand, side-shifting or peeling of the first fiber composite form the end and, on the other hand, protects the first fiber composite from damage. In accordance with the invention, a further layer of material, more particularly, a protective layer and/or sliding layer, may be preferably applied to the second fiber composite protecting the fibers highly sensitive to transverse compression, whilst providing adequate sliding properties in telescopic extension and retraction and, more particularly, creating optimized conditions regarding exposure to the sun.
A telescopic jib, in accordance with the invention, finding application more particularly on a crane or mobile crane, comprises an articulately jointed base section and at least one telescopic section; and is configured so that at least one of the sections is configured as the telescopic part in accordance with the description and embodiments as discussed above.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.